how to build 64bit xapian using MSVC2017?

[Olly Betts]

On Tue, Mar 20, 2018 at 06:30:07PM +0000, Olly Betts wrote:
> https://lists.xapian.org/pipermail/xapian-discuss/2018-January/009585.html

Related to this, the appveyor build is currently failing on git master.
Unfortunately the change at which is started to fail was the addition
of the new "honey" backend, which doesn't narrow things down to a
useful degree.

I've checked over the new code and turned up a few issues, but fixing
those didn't solve this.

It's failing for MSVC and mingw (but not cygwin) so I'm suspecting it
is something to do with the C runtime (since mingw uses MSVC's C
runtime DLL).  If someone who actually cares about this platform is
interested in helping that'd be great

Cheers,
    Olly